As shown in FIG. 1, a conventional flyback converter 10 includes a controller chip 12 to provide a control signal VGATE to switch a power switch SW serially connected to a transformer 14, so as to convert an input voltage VIN into an output voltage VOUT for a load RL, an opto-coupler 16 to generate a feedback signal Vcomp according to the output voltage VOUT for the controller chip 12, and a current sense resistor Rcs serially connected to the power switch SW to provide a current sense signal Vcs, which is a function of the primary current Ip of the transformer 14, for the controller chip 12. The controller chip 12 determines the control signal VGATE according to the feedback signal Vcomp and the current sense signal Vcs. For the flyback converter 10, the input voltage information at the power input VIN is very important because it is useful in controlling the maximum output power of the flyback converter 10. In addition, the input voltage VIN also has great impact on the load point at which the flyback converter 10 enters frequency reduction mode or burst mode control.
FIG. 2 is a diagram showing the burst mode entry points under different input voltages VIN. When the load RL transits to light, the load current IL supplying for the load RL begins to fall down, as shown by the waveform 20, and in consequence the feedback signal Vcomp decreases accordingly. The flyback converter 10 will enter a burst mode when the feedback signal Vcomp becomes lower than a preset level 26. Actually, due to the propagation delay and slope compensation, the variation of the input voltage VIN will change the time point at which the flyback converter 10 enters the burst mode. More particularly, when the input voltage VIN is higher, the feedback signal Vcomp will have a lower level, as shown by the waveform 24, so that the flyback converter 10 enters the burst mode earlier, which may lead to audio noise. On the contrary, when the input voltage VIN is lower, the feedback signal Vcomp will have a higher level, as shown by the waveform 22, and consequently, the flyback converter 10 enters the burst mode later and may have poor efficiency. It is therefore needed compensation by the input voltage information.
On the other hand, minimum on-time control is useful to reduce the switching frequency of the power switch SW under no load or light load. However, for a fixed minimum on-time, the output ripple becomes large under light load at high input voltage. During low input voltage, energy transfer of each switching is so small and cannot reduce the switching frequency much. This can be solved by modulating the minimum on-time by the input voltage information.
There are two common methods to obtain the input voltage information, as shown in FIGS. 3 and 4. As shown in FIG. 3, the first one is to insert serially connected resistors R1 and R2 between the power input VIN and the controller chip, to produce a current I1 containing the input voltage information, while the other one, as shown in FIG. 4, uses voltage divider resistors R1, R2 and R3 at the power input VIN to obtain the input voltage information. However, both of them need extra pin Vinsense and external components for sensing the input voltage VIN. If the controller chip 12 shown in FIG. 1 has six-pin package, all the six pins thereof have be used so that no blank pins are available for sensing the input voltage VIN. Eight-pin package may be used instead, but it costs more than that of six-pin package. Moreover, for some SOP-8 applications, there is a pin left for high voltage startup device and an adjacent pin thereof is cut to leave enough safety distance. The other six pins are the same assignment as the six-pin package, so there is no extra pin for sensing the input voltage VIN neither.
Therefore, it is desired a solution to obtain the input voltage information without extra pins.